Multi-channel structurally robust brain probe and method of making the same

ABSTRACT

A bio-probe having a base and a tip, and comprising a core of substantially rigid, high-strength material, said core tapering inwardly from the base to the tip and a set of conductors extending longitudinally about said core. In addition, dielectric material, substantially electrically isolates each conductor from its surroundings. Also, a set of apertures is defined by the dielectric material to the set of conductors, thereby defining a set of electrodes.

STATEMENT OF GOVERNMENTAL SUPPORT

This invention was made with government support under grant No.1R43MH59502-01 awarded by the Small Business Research Program of theDepartment of Health and Human Services of the Public Health Service.The government has certain rights in the invention.

BACKGROUND OF THE INVENTION

The assembly of a brain probe assembly employed in brain research isquite challenging from both a structural and an electrical standpoint.

Structurally, probes must not fray or in any way come apart when pushedthrough the dura, a tough membrane covering the brain, and other braintissue. Probe should have enough strength and rigidity to broach thedura without the need for assistance by, for example, a guide tube or aninitial incision.

Moreover, probes must not break, running the risk of leaving a fragmentin the brain. Also, they must not cause undue damage to tissue at thesensing site. Inevitably, the tissue separating the sensing site fromthe brain exterior will suffer some damage as a probe is pushed to itsdestination.

Electrically, one should note that field signals to be detected in thebrain, are typically of the order of 100 to 500 μvolts. The lowamplitude of these signals makes it necessary to amplify them asphysically close as possible to their source. In fact, the signalsinvolved are so minute that variations in circuit geometry could wellaffect significantly the detection processing of the signals. It is alsohighly desirable to minimize cross-talk between any two signals. Giventhe tight geometries allowable for brain probe design, theserequirements are difficult to meet simultaneously.

SUMMARY OF THE INVENTION

In a first separate aspect the present invention is a bio-probe having abase and a tip, and comprising a core of substantially rigid,high-strength material. The core tapers inwardly from the base to thetip and a set of conductors extend longitudinally about the core. Inaddition, dielectric material, substantially electrically isolates eachconductor from its surroundings. Also, a set of apertures are defined bythe dielectric material to the set of conductors, thereby defining a setof electrodes.

In a second separate aspect, the present invention is a method ofproducing a bio-probe. This method includes the step of providing atapering core of substantially rigid material. The core is then coatedwith dielectric material and this dielectric material is coated with afirst layer of conductive material. The conductive material is thendivided into longitudinal traces, extending from the base into proximityto said tip. The conductive material is then coated with a second layerof dielectric material. Finally, portions of the second layer ofdielectric material are removed to form apertures to the conductivematerial, thereby forming electrodes.

In a third separate aspect, the present invention is a bio-probeassembly for measuring bio-electrical signals, comprising, a probeportion having a distal end and a proximal end and a set of electrodesat said distal end for detecting the bio-electrical signals. Eachelectrode is connected to a longitudinal conductor, extending to theproximal end and a set of substantially identical amplifier circuitcards connected to the longitudinal conductors. Accordingly, eachbio-electrical signal is amplified in substantially the same manner.

The foregoing and other objectives, features and advantages of theinvention will be more readily understood upon consideration of thefollowing detailed description of the preferred embodiment(s), taken inconjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exploded perspective view of bio-probe assembly accordingto the present invention.

FIG. 2 is a front view of the circuit card assembly of the bio-probeassembly of claim 1.

FIG. 3 is an expanded perspective view of the tip of the bio-probeassembly of FIG. 1.

FIG. 4 is a greatly expanded cross-sectional view of the tip of thebio-probe assembly of FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A preferred embodiment of a brain probe assembly 10, according to thepresent invention is composed of a probe core 12 and a handle core 14.The probe core 12 is made of tungsten, chosen for its material stiffnessand tensile strength. Probe core 12 must be absolutely straight. Toachieve this end, a straightening machine that pulls on core 12, therebycreating tensile stress and annealing core 12 may be used. A tip ordistal end 20 of probe core 12 has a diameter of 200 microns (8.0 mils)and a base or proximal end 24 of core 12 has a diameter of 600 microns(24 mils). In addition, core 12 is 89 mm (3.5″) long. The tip 20 ispreferably formed by way of centerless grinding. Probe core 12 should beelectro polished so that the deposition of materials onto it (see below)can be accomplished efficiently and so that the finished assembly 10 canpass through brain tissue as smoothly as possible.

For ease of assembly and so that operating personnel may more easilyhandle assembly 10, the handle core 14 is expanded in cross-sectionrelative to probe core 12. Although the handle core 14 is preferably aunitary piece of medical grade 304 stainless steel, it may beconceptually divided into a cylinder 15, having a diameter of 4.826 mm(0.19″), and a frustum 17. The frustum 17 tapers inwardly at 15° anglefrom the sides of cylinder 15. A 600 μm (24 mil) aperture (not shown) atthe narrow end of frustum 17 permits introduction of the base of probecore 12, after which probe core 12 is joined to handle core 14, by wayof an epoxy, to form joint core 26. The epoxy used must be conductive,so that the probe core 12 is grounded to the base core 14, heatresistant, so that it withstands the sterilization process that theprobe 10 must undergo in use. It must also be able to withstand thedifferent degrees of expansion that stainless steel and tungsten undergoduring the sterilization process. An epoxy that is available from EpoxyTechnology, Inc. of Billerica, Mass. under the designation E3084 appearsto meet these requirements. In an alternative preferred embodiment, theprobe core 12 is laser-welded to the base core 14.

After joint core 26 is produced, it is dip coated with a dielectricepoxy, which has been premixed with a surfactant to promote an evencoating, to form an insulating coat 30. The desirable characteristicsfor an epoxy to be used are biocompatibility, heat tolerance towithstand the sterilization process, low viscosity to produce a thinfilm, a heat accelerated cure and a high bulk resistivity and a lowdielectric coefficient to avoid electrical losses and withstandelectrostatic charges. One epoxy that appears to meet these requirementsis available as #377 from Epoxy Technology, Inc. of Billerica, Mass. Asuitable surfactant is available as FC-430 from 3M of St. Paul, Minn. Inan additional preferred embodiment quartz crystal, glass or a similardielectric material is vacuum deposited to form coat 30. In thispreferred embodiment, in order to gain adherence, however, a 200 Å coatof chrome (not shown) is first applied, also through vacuum depositionon core 26 to promote the adhesion of coat 30. The thickness of coat 30is chosen to minimize the capacitance between core 26 and the conductivetraces 50 (see below) deposited over it.

On top of coat 30, a 0.5 μm thick plate of conductive material (notshown as such but later rendered into a set of traces 50) is,preferably, vacuum deposited. This plate 50 also may be adhered by wayof a 200 Å layer of vacuum deposited chrome (not shown). Plating 50 mustbe highly conductive and, if vacuum coating is used, must be an elementof the periodic table. Accordingly, gold, platinum and iridium are amongthe materials that may be used. Other deposition techniques, such aschemical deposition, may permit the application of other highlyconductive materials, such as a conductive polymer. The material used tocreate plating 50 must also be susceptible to removal by laser ablatingor an etching process.

Next, plate 50 is sectioned into 24 longitudinal traces 50 (othernumbers of traces 50 are possible) extending from approximately the tip20 to the proximal end of base core 14. Accordingly, near the tip 20 thetraces 50 have a pitch of about 27 μm, near the base 24 have a pitch ofabout 80 μm at the proximal end of handle 14 have a pitch of about 630μm. Of particular utility for performing task of sectioning theconductive plate into traces 50 is a frequency multiplied ND:YAG laser,which can cut kerfs to separate the traces on the order of 5-10 μmwidth.

In one preferred embodiment there are just four traces 50. Using thisembodiment a compound probing device may be built that incorporates anarray of probe assemblies 10 to sense and or stimulate a number ofneural sites separated not just in depth, but also transversely to probeassembly 10 longitudinal dimension.

Next, the conductive traces 50 are coated with an outer layer 60 of highcoefficient dielectric material. An additional dip coat of epoxy #377 isone way of accomplishing this. Another method is a vacuum deposition ofglass or quartz crystal placed, again over an intermediate 200 Å layerof chrome. Dielectric layer 60 preferably has a thickness of from 10 to40 um to avoid damage by static electric discharge. A laser is used toablate this outer layer to create several apertures extending throughlayer 60, having a diameter of about 10 μm at each prospectivemicroelectrode site. A platinum-iridium electrode 62 is built up,preferably by electroplating, at each of these sites.

Base 14 is attached to a plate 70 that includes outwardly extendingconductive traces (not shown) that connect traces 50 to a set ofconnector pins 72. In turn a set of connectors 72 on plate 70 attach toa matching set of connectors 74 on a circuit card assembly 80. Assembly80 includes a set of 24 circuit cards 82, one for each trace, eachbearing an identical amplification circuit for processing each signalfrom each trace 50 in an identical manner.

The advantages of the present invention should now be apparent. Probeassembly 10 is strong, smooth and sleek, for moving through brain tissueto the site of interest. The cross capacitance between traces 50 isminimized due to the shape of the traces 50, which are curved solidrectangles, on the order of 0.5 μm thick but varying between 10 μm and50 μm wide. Finally, identical circuits 82 ensure equal treatment foreach trace signal.

The terms and expressions that have been employed in the foregoingspecification are used as terms of description and not of limitation.There is no intention, in the use of such terms and expressions, ofexcluding equivalents of the features shown and described or portionsthereof, it being recognized that the scope of the invention is definedand limited only by the claims which follow.

What is claimed is:
 1. A bio-probe having a base and a tip, andcomprising: (a) a core of substantially rigid, high-strength material,said core being substantially circular in cross-section, and tape ringinwardly from said base to said tip, said tapering being substantiallyuniform in cross-section so that said tip of said core has substantiallythe same cross-sectional shape as said base of said core, but is of asmaller cross-sectional size; (b) a set of conductors extendinglongitudinally from said base to a position near to said tip andcollectively substantially circumscribing said core; (c) dielectricmaterial, substantially electrically isolating each said conductor fromits surroundings; and (d) a set of apertures through said dielectricmaterial to said set of conductors, hereby defining a set of electrodes.2. The bio-probe of claim 1, wherein said dielectric material is dividedinto a first layer of dielectric material and a second layer ofdielectric material and wherein said first layer of dielectric materialis deposited directly upon said core.
 3. The bio-probe of claim 2,wherein said second layer of dielectric material is made of epoxy resin.4. The bio-probe of claim 2, wherein said set of conductors are madefrom conductive material that is deposited directly upon said firstlayer of dielectric material.
 5. The bio-probe of claim 4, wherein eachconductor of said set of conductors is roughly rectangular in crosssection and is each more than three times as wide as it is thick and issubstantially conformal over said first layer of dielectric material. 6.The bio-probe of claim 4, wherein each pair of conductors of said set ofconductors is mutually separated by a trench running longitudinallyalong the length of said bio-probe.
 7. The bio-probe of claim 1, whereinsaid core is comprised of tungsten.
 8. The bio-probe of claim 1, whereinsaid apertures are filled with conductive material.
 9. The bio-probe ofclaim 1, wherein said dielectric material has an exterior surface andsaid apertures are filled with conductive material that extends onto andbeyond said exterior surface of said dielectric material.
 10. Thebio-probe of claim 1, wherein said core, said set of conductors and saiddielectric material form a first portion that has a base and furtherincluding a second portion having a tip and a base and wherein said tipof said second portion is positively attached to said base of said firstportion.
 11. The bio-probe of claim 1, wherein said core is smoothlycircular in cross-section.
 12. The bio-probe of claim 1, wherein saidcore has quadrilateral symmetry in cross-section.